Do you have to have 4g do you mean? Or do you mean can they magically input the workings into a non4G phone?
If the first nah you can stick with 3g if the latter can I have some of what you are smoking? And no you won't be able to upgrade a 1080p screen to 4k in a few years without buying a new tv either.

From EE:
"Customers can either wait for EE to launch (we will be announcing the date in the coming weeks) or they can sign up to a contract with T-Mobile or Orange and upgrade to EE free of charge once it has launched, as long as they move to an equivalent priced and length plan on EE."